Solution:
Within the "Solution" (Blogengine.Core) i found the BlogSettings.cs
at Line 1112 i have to Change the value:
public bool RemoveExtensionsFromUrls { get { return true; } }
to:
public bool RemoveExtensionsFromUrls { get { return false; } }

You have to download the Code Version (https://github.com/rxtur/BlogEngine.NET)
After you open the solution with visual Studio (There is a "free" Community Version) you could find the file within the Core Folder.
But you Need to compile the solution and then replace the files. Ist recommend to make a copy of your blog.